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Mobile radio hum

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dixie729

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Installed a CDM 1250 in a Ford Interceptor Sedan. Used the upfitter wiring harness from the car. When transmitting, a humming sound is heard on the receiving side when the car is running. Also can hear the siren through the radio while transmitting.
Thought maybe it could have been the coax so it was replaced with a new nmo style mount and cable but the problem is still there.
Any ideas?

Kevin
 

mmckenna

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Which wiring did you tap into? There should be a large lead that comes off the battery, your radio should be attached to that.
Run your radio negative lead to body ground. In addition to that, ground the radio chassis to the body. Yes, I know the radio is grounded through the negative power lead and the antenna coax shield. You want the radio chassis grounded.

I'd also take a close look at the alternator and make sure it's not starting to go. You can put a digital multimeter on the power feed to the radio and set it to AC volts. It should read down in the millivolt AC range.

Also, if you are running a CTCSS tone on the radio TX side, and it's one of the higher frequency tones, you can sometimes get that come through on the receiving radio audio path if it's a cheap radio or doesn't have a pass filter on the audio path.
 

KE0VUL

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I'd also check the battery. Had one of those go not long after weird noises, I tapped into the lighter plug though. I've since went directly to the battery and chassis ground points. Little more work but so is routing the antenna wires in the first place.
 

dixie729

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Which wiring did you tap into? There should be a large lead that comes off the battery, your radio should be attached to that.
Run your radio negative lead to body ground. In addition to that, ground the radio chassis to the body. Yes, I know the radio is grounded through the negative power lead and the antenna coax shield. You want the radio chassis grounded.

I'd also take a close look at the alternator and make sure it's not starting to go. You can put a digital multimeter on the power feed to the radio and set it to AC volts. It should read down in the millivolt AC range.

Also, if you are running a CTCSS tone on the radio TX side, and it's one of the higher frequency tones, you can sometimes get that come through on the receiving radio audio path if it's a cheap radio or doesn't have a pass filter on the audio path.
No wiring was tapped. There are 3 main power and 3 ground leads that go to the battery and fused and come into the car. It is a police package car, all the wiring is ran in the car. It's hooked up to one of the power and one of the grounds inside the car.
I had this exact problem with this same radio in a different car so I took wires directly to the battery and it still had the hum in the other car.
I forgot to mention that it doesn't make the hum when the car is off. I'll check the alternator when I get a chance.
It's a PL tone on TX.
CDM1250 vhf
 

12dbsinad

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What else is in the car that powers up when you start it? Is there something common with the other car you had the radio in? Start pulling fuses to all your other aftermarket accessories and see if it goes away. Also make sure your grounds aren't all grouped together like so many upfitters love to do... causes ground loops, ground the radio by itself.
 
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